Generic characters. Animal oval, lobes of the mantle simple, united poste- 

 riorly and terminating in two short syphons, joined at their base, without 

 tentacles ^ mouth oval shaped, small ; tentacles of the mouth short and nar- 

 row ; gills rather broad, nearly equal, united behind the foot ; foot narrow, 



el s n hen e oval, nearly equilateral; beaks somewhat inflated and prominent ; 

 hin^e margin narrow, with two primary teeth in each valve ; lateral teeth 

 elongated; palleal impression simple; ligament external, narrow, situated on 

 the longer portion of the shell. 



Description of species.* 

 a. Species with rounded but not protuberant beaks. 



1. Sph. sulcatum, Lam. 



Cyclas sulcata, Lam., An. s. vert. v. 560, 1818. 

 C. Saratogea, Lam., loc. sub. cit. v. 560, 1818. 

 C. similis, Say, Nich. Encycl. Amer. edit. ix. pi. 1, fig. 9, 1818. 

 d. lasmampsis, Rafin., II. scie. Phys. ix. 319, pi. 82, f. 19, 20, 1820. 

 C. solida, DeKay, Kept. 220, pi. xxv. f. 265, 1842. 

 C. gigantea, Prime, Host. Proc. iv. 157, 1851. 

 C. ponderosa, Prime, loc. sub. cit. iv. 157, 1851. 

 C. striatina, Lam., Fer. in Mag. Zool., 1835. 

 C. rhomboidea, Say, C. B. Adams, Vermont cat. 18, 1842. 

 Animal white, tubes a light orange color. 



Shell transversely oval, nearly equilateral, light in texture for its size ; pos- 

 terior margin somewhat more pointed ; anterior rounded, base slightly curved: 

 valves convex ; beaks full, raised above the outline of the shell ; posterior 

 portion a little longer ; sulcations coarse, regular ; epidermis dark chestnut 

 brown ; interior light blue ; hinge margin narrow, nearly a straight line ; 

 cardinal teeth small, indistinct, situated somewhat towards the anterior side, 

 double in both valves, and so placed as to assume the shape of the letter V 

 reversed ; lateral teeth on a line with the primary teeth, large, strong and 

 prominent. 



The young is more equilateral than the adult ; more compressed ; it pre- 

 sents the shape of a quadrilateral, it is of a light lemon color, the striations 

 are as heavy as those of the mature shell. 

 Long. 11-16 ; lat. 71-61 ; diam. 5-16 inches. 



Hab. North America, in the New England States, and in the States of 

 New York, New Jersey, Pennsylvania, Ohio, Michigan, Wisconsin and Ala- 

 bama. 



(Cabinet Acad. of Nat. Sci. of Phila., Garden of Plants in Paris, Mus. De- 

 lessert, Jay, Prime and others.) 



This, our most common and widely distributed species, living as it does in 

 so many different sections of the country, presents at times great variations 

 in size, color and general appearance. It can, however, be easily recognized 

 by its very elongated and equilateral form, and by the beaks which are uni- 

 formly full and convex ; they are often eroded. The young is often of an 

 uniform light lemon color, which, as the shell matures, becomes gradually 

 darker from the beaks downwards until the new shade covers the whole sur- 

 face of the shell ; in certain intermediate stages of growth, the shell is marked 

 with a zone of yellow on the inferior margin ; the color of the adult varies 

 from a greenish brown to a dark chestnut. The young shell has at times, 

 owing to the variations which exist between it and the adult, been taken for a 

 different species ; by some it has been taken for the Sph. rhomboideum. 



* I ana preparing for publication, as a complement to this Monagraph, an Atlas with 

 colored figures of the different species described. 



